Eleven families of phosphodiesterases (PDEs) have been identified but only PDEs in Family I, the Ca2+-calmodulin-dependent phosphodiesterases (CaM-PDEs), have been shown to mediate both the calcium and cyclic nucleotide (e.g. cAMP and cGMP) signaling pathways. The three known CaM-PDE genes, PDE1A, PDE1B, and PDE1C, are all expressed in central nervous system tissue. PDE is expressed throughout the brain with higher levels of expression in the CA1 to CA3 layers of the hippocampus and cerebellum and at a low level in the striatum. PDE1A is also expressed in the lung and heart. PDE1B is predominately expressed in the striatum, dentate gyrus, olfactory tract and cerebellum, and its expression correlates with brain regions having high levels of dopaminergic innervation. Although PDE1B is primarily expressed in the central nervous system, it may be detected in the heart. PDE1C is primarily expressed in olfactory epithelium, cerebellar granule cells, and striatum. PDE is also expressed in the heart and vascular smooth muscle.
Cyclic nucleotide phosphodiesterases decrease intracellular cAMP and cGMP signaling by hydrolyzing these cyclic nucleotides to their respective inactive 5′-monophosphates (5′AMP and 5′GMP). CaM-PDEs play a critical role in mediating signal transduction in brain cells, particularly within an area of the brain known as the basal ganglia or striatum. For example, NMDA-type glutamate receptor activation and/or dopamine D2 receptor activation result in increased intracellular calcium concentrations, leading to activation of effectors such as calmodulin-dependent kinase II (CaMKII) and calcineurin and to activation of CaM-PDEs, resulting in reduced cAMP and cGMP. Dopamine D1 receptor activation, on the other hand, leads to activation of nucleotide cyclases, resulting in increased cAMP and cGMP. These cyclic nucleotides in turn activate protein kinase A (PKA; cAMP-dependent protein kinase) and/or protein kinase G (PKG; cGMP-dependent protein kinase) that phosphorylate downstream signal transduction pathway elements such as DARPP-32 (dopamine and cAMP-regulated phosphoprotein) and cAMP responsive element binding protein (CREB). Phosphorylated DARPP-32 in turn inhibits the activity of protein phosphatase-1 (PP-1), thereby increasing the state of phosphorylation of substrate proteins such as progesterone receptor (PR), leading to induction of physiologic responses. Studies in rodents have suggested that inducing cAMP and cGMP synthesis through activation of dopamine D1 or progesterone receptor enhances progesterone signaling associated with various physiological responses, including the lordosis response associated with receptivity to mating in some rodents. See Mani, et al., Science (2000) 287: 1053, the contents of which are incorporated herein by reference.
CaM-PDEs can therefore affect dopamine-regulated and other intracellular signaling pathways in the basal ganglia (striatum), including but not limited to nitric oxide, noradrenergic, neurotensin, CCK, VIP, serotonin, glutamate (e.g., NMDA receptor, AMPA receptor), GABA, acetylcholine, adenosine (e.g., A2A receptor), cannabinoid receptor, natriuretic peptide (e.g., ANP, BNP, CNP), DARPP-32, and endorphin intracellular signaling pathways.
Phosphodiesterase (PDE) activity, in particular, phosphodiesterase 1 (PDE1) activity, functions in brain tissue as a regulator of locomotor activity and learning and memory. PDE1 is a therapeutic target for regulation of intracellular signaling pathways, preferably in the nervous system, including but not limited to a dopamine D1 receptor, dopamine D2 receptor, nitric oxide, noradrenergic, neurotensin, CCK, VIP, serotonin, glutamate (e.g., NMDA receptor, AMPA receptor), GABA, acetylcholine, adenosine (e.g., A2A receptor), cannabinoid receptor, natriuretic peptide (e.g., ANP, BNP, CNP), endorphin intracellular signaling pathway and progesterone signaling pathway. For example, inhibition of PDE1B should act to potentiate the effect of a dopamine D1 agonist by protecting cGMP and cAMP from degradation, and should similarly inhibit dopamine D2 receptor signaling pathways, by inhibiting PDE1 activity. Chronic elevation in intracellular calcium levels is linked to cell death in numerous disorders, particularly in neurodegenerative diseases such as Alzheimer's, Parkinson's and Huntington's Diseases and in disorders of the circulatory system leading to stroke and myocardial infarction. PDE1 inhibitors are therefore potentially useful in diseases characterized by reduced dopamine D1 receptor signaling activity, such as Parkinson's disease, restless leg syndrome, depression, narcolepsy and cognitive impairment. PDE1 inhibitors are also useful in diseases that may be alleviated by the enhancement of progesterone-signaling such as female sexual dysfunction.
PDE2 also hydrolyzes both cAMP and cGMP with a high Vmax and low Km. The enzyme is allosterically stimulated by cGMP binding to one of its GAF domains. Only one gene family, PDE2A, codes for the PDE2 with three splice variants of PDE2, namely PDE2A1, PDE2A2 and PDE2A3. PDE2A1 is localized in the cytosol while PDE2A2 and PDE2A3 are membrane bound. PDE2 has been shown to have therapeutic potential in neuronal development, learning and memory. Van Staveren et al., Brain Res. (2001) 888:275; O'Donnell et al., J. Pharm. Exp. Ther. (2002) 302:249; Velardez et al., Eur. J. Endo. (2000) 143:279; Gallo-Paget et al., Endo. (1999) 140:3594; Allardt-Lamberg et al., Biochem. Pharm. (2000) 59:1133; and Wakabayashi et al., Miner. Res. (2002) 17:249. Inhibition of PDE2A demonstrates enhanced cognitive function across multiple preclinical models of cognitive performance that reflects improvements in recognition memory, social interaction and working memory, which are all deficient in schizophrenia. Boess et al., Neuropharmacology (2004) 47(7):1081-92. PDE2A inhibition also improves cognitive deficits that develop in aging and Alzheimer's disease. Domek-Lopacinska et al., Brain Res. (2008) 1216:68-77; Brandon et al., Annual Reports in Medicinal Chemistry (2007) 42:4-5. PDE2A inhibition has also been demonstrated to show efficacy in preclinical models of anxiety and depression. Masood et al., JPET (2009) 331(2):690-699; Masood et al., JPET (2008) 326(2):369-379; Reierson et al., Neurosci. Lett. (2009) 466(3):149-53.
PDE1 and PDE2 have been shown to be useful therapeutic targets. There is thus a need for a product that comprises a compound that selectively inhibits PDE1 activity and a compound that selectively inhibits PDE2 activity, in free or salt form.